Modems are frequently used in paging systems to transmit data over telephone lines or by radio transmission between a central paging terminal and a plurality of paging transmitters, which re-transmit the data as a radio signal to a pager unit carried by an individual subscriber to the paging service. The data received by the paging unit of the intended recipient can then be read by the subscriber on a display screen that is part of the pager unit. To ensure that the data signal reaches the individual's pager unit anywhere in a defined geographic coverage area, the plurality of paging transmitters in the system have overlapping reception zones, and the same data signal is simultaneously broadcast from each of the paging transmitters. Paging systems with this type of simultaneous zone coverage are referred to as simulcast paging systems.
When first developed, simulcast paging systems typically transmitted data to subscribers at a relatively low rate, e.g., 512 or 600 baud. Subsequently, the increased demand on paging systems for higher data throughput as greater numbers of pagers have been placed into service has prompted an increase in the data rate of the modems used to transfer data between the central paging terminal and the paging transmitters to 1,200 baud. Integrated circuit modem chipsets conforming to the Bell 202 or the V.23 standards are readily available to achieve this level of performance. However, there is now a demand for paging modems that can operate at still higher baud rates, e.g., at or above 2,400 baud, because simulcast paging systems are being forced to access still more customers over a limited number of radio channels. It is therefore critically important that each data message be transmitted as rapidly as possible to enable more customers to be serviced. Unfortunately, asynchronous paging modems capable of operating at the higher baud rates are not presently available. Conventional modems are limited in speed, at least for this type of application, for several technical reasons, the most important of which are their inherent isochronous distortion level and maximum data rate limitation of about 1,500 baud.
To achieve data rates in excess of about 1,200 baud, an asynchronous modem suitable for use in a simulcast paging system must be capable of operation at a relatively low isochronous distortion level. Isochronous distortion level is a measure of the difference between the absolute earliest and the absolute latest transitions in a digital data stream, where the transitions are compared to ideal sample times. This type of distortion is thus affected by the time deviation between changes in data logic level (between logic level one and zero for each bit) for the same data signal being transmitted by simulcast paging transmitters having overlapping reception zones. This parameter is also referred to as system jitter. System jitter is an important operating parameter of a simulcast paging system because of its effect on the signal received by an individual's pager unit in the overlapping reception zone of two or more adjacent simulcast paging transmitters. To ensure that the pager unit receives the data signal accurately, the overlapping transmissions of the paging transmitters must be aligned to within approximately one-quarter of a bit time. For example, if the data is transmitted at 2,400 baud, the data edges transmitted by the various paging transmitters must be aligned with respect to each other to within about 104 .mu.s. A typical maximum overlap area located about ten miles farther from one paging transmitter than the other would contribute approximately 54 .mu.s misalignment (based on the radio signal propagating over a distance of one mile in about 5.4 .mu.s at the speed of light), leaving about 50 .mu.s peak-to-peak (or .+-.25 .mu.s peak) for non-alignment caused by system jitter. Similarly, at 1,200, 600, and 512 baud, the maximum allowable peak system jitter for this propagation condition is about .+-.75, .+-.180, and .+-.210 .mu.s, respectively. Absolute delays caused by the transmission time between the paging terminal and the paging transmitters of the simulcast paging system are another potential source of differences in the time that bit changes are received by a paging unit in an overlap area, but this source of delay is readily compensated by the system by appropriately delaying the radio transmission from the paging transmitter that is first to receive the data from the central paging terminal, as is well known to those of ordinary skill in the art.
A conventional asynchronous modem of the type used in simulcast paging systems processes non-return-to-zero (NRZ) data with a typical peak system jitter of about .+-.50 to .+-.60 .mu.s under optimum laboratory test conditions on a back-to-back modem link, with no noise. Conventional modem chip sets, such as the Texas Instruments.TM. Type TCM 3105, will not even operate above about 1,500 baud under the 15-20 dB SINAD (signal-to-noise-and-distortion) conditions typical of a paging system link. It will therefore be apparent that prior art simulcast paging system modems are generally limited to about 1,200 baud. What is needed is a relatively low-cost modem that can be used in a simulcast paging system at data rates well above 1,200 baud without introducing excessive system jitter that would disrupt the reception of transmitted data in an overlap area.
Further, a suitable modem for use on a simulcast paging system must include a modulator that is capable of modulating a signal for transmission to a demodulator over a bandwidth of about 3 KHz, thereby insuring its compatibility with existing telephone and radio systems. Frequency shift keyed (FSK) modulators are conventionally used for this purpose. Jitter is particularly a problem for a demodulator processing a modulated signal transmitted over a bandwidth of 3 KHz. Conventional FSK demodulators use auto-correlation or phase-locked loop techniques to determine the binary bits corresponding to a modulated signal. These techniques are inherently too slow to be used on modems operating at data rates above about 1,200 baud.
One of the problems with existing FSK modems operating at data rates above about 1,200 baud, with a bandwidth limited to about 3 KHz, is the aliasing that occurs between positive and negative frequencies used in the FSK signal. The band for the FSK modulated signal is normally centered at about 1,700 Hz and the FSK signal is typically modulated between 1,200 and 2,200 Hz to convey binary data comprising ones and zeroes. When the FSK modulation frequencies are generated directly in the desired frequency band of the transmitted FSK modulated signal, the skirts of the positive frequencies and their corresponding negative frequencies in the frequency spectrum overlap, causing the aliasing problem. Accordingly, a modulator is required for use in a simulcast paging system modem that removes the aliasing of the FSK frequencies from the modulated FSK signal.